bug: dubious typing of record literals with variant fields

[crusso]

Moc currently accepts:

  let x : { var v : Int } = {
    var v : Nat = 1;
  };

and correctly rejects:

  let x : { var v : Nat } = {
    var v : Nat = 1;
  };
  let y : { var v : Int } = x;

The former is dubious, but I can't actually get it to go wrong because there is no way to alias the field v at both types.

Morally, this example would demonstrate unsoundness:

  let x : { var v : Int; var w : Nat} = {
    var v : Nat = 1;
    alias w = v
  };

Since x.v := -1 would (also) assign -1 to x.w, a Nat.

But I don't see how to emulate this, so perhaps we are safe after all.